This invention relates generally to cathode structures and more particularly to cathode structures exhibiting secondary emission.
As is known in the art, certain microwave tube devices such as high power crossed-field amplifiers generally include a cathode which is capable of supporting secondary emission of electrons and thus which is capable of providing high current density and high power capabilities.
One type of conventional cathode structure capable of secondary emission generally includes a metal which is capable of exhibiting sufficient secondary emission. Examples of metal materials which are often used as secondary emission cathodes include platinum and tungsten. These materials have a ratio of secondary emission to primary emission which is suitable for some applications, although for most applications the ratio is generally too low.
Another type of material which is suitable for secondary emission is certain metallic dielectric oxides such as beryllium oxide, magnesium oxide, and aluminum oxide. These insulator type of oxides exhibit reasonably high ratios of secondary emission to primary emission. One problem with these materials, however, is that they cannot be used in bulk form since they are electric insulators. Since they are insulators, impinging electrons charge the surface of the metallic oxide effectively stopping the secondary emission process. One approach to overcome this limitation is to provide cathodes made of a metal having a thin film of such oxides thereover with the film having a thickness of approximately 50.ANG.. Thin films on the order of 50.ANG. disposed over the metal cathode permit impinging electrons to tunnel through the insulative film and be collected by the metal cathode structure. With the metallic oxide layer the composite cathode is capable of providing a high current density (approximately 1 to 10 amperes per square centimeter). Therefore these films are suitable for use as secondary emission cathodes in crossed-field high power tubes.
One problem with this approach, however, is that since these films are very thin, the thin oxide films are eroded away by the electron bombardment over a relatively short period of time. Therefore such cathodes have a limited lifetime in applications as a cathode in a crossed-field high-power amplifier tube. As mentioned above, the process by which charge is leaked off of these thin films is to the film thin such that tunneling of impinging primary electrons can be provided through the film to the conductive electrode which supports the film. One approach which has been used to overcome the problem of erosion of these thin films is to provide the conductive electrode as a layer of the metal used in the selected secondary emitter metallic oxide layer and to also manufacture the tube with a in-situ oxidizer which permits reformation and rehealing of the oxide film during tube operation. This approach while acceptable for tube operation is, nevertheless, costly to incorporate into the tube. Further, the in-situ approach occupies space in the tube. An additional problem with these films is that the films require an extensive period of time for out-gassing of impurities during manufacture of the tube in order to allow them to be used at high powers.
Thus, in order to increase the longevity of the cathode, but not necessarily improve the out-gassing problem, thicker films for the cathode would be desired. Thicker films, however, are not readily useable since the thicker films will introduce problems with respect to the effective conductivity of the composite cathode which will result in the charging effects within the films, as mentioned above, and thus provide a reduction of the available current density relative to that obtained from the very thin insulating films.
One solution to this problem has been to obtain greater electronic conduction in such thick insulating films by introducing conductive metallic particles into the insulating film. In particular, one example of such a material is magnesium oxide containing gold particles. The metallic particles result in improved conductivity of the material, however, the presence of the metallic particles also provides a significant degradation in the secondary emission ratio. Moreover, the slight increase in thickness allowed by the addition of metallic particles does not adequately meet the requirements for a cathode having a relatively long lifetime.